1. Field of the Invention
The present invention generally relates to a motor controller and more particularly relates to an apparatus for performing an accurate positioning control by using a DC motor of a small size and a method of driving a DC motor.
2. Description of the Related Art
A DC motor including a brush and a commutator has a simple structure and can be manufactured at a low cost. Also, a DC motor achieves high efficiency and high output although its size is small, and needs no special driver. For these reasons, a DC motor is now used in numerous appliances.
However, if it is necessary to control the stop angle of the rotor of a DC motor accurately enough or to rotate the motor at an extremely low number of revolutions per minute, it might be difficult for a DC motor to satisfy these requirements fully.
This is partly because a DC motor generates a cogging torque. Hereinafter, the cogging torque of a DC motor will be described.
As shown in FIG. 8A, a DC motor includes a rotor 94 and fields 95. The rotor 94 includes magnetic poles 91, 92 and 93, each of which includes an iron core made of a magnetic material such as silicon steel and a coil that is wound around the iron core. Each of the fields 95 is a permanent magnet such as a ferrite magnet. The DC motor typically has three magnetic poles 91, 92 and 93 and two fields 95 as shown in FIG. 8A.
In a DC motor like this, the magnetic poles 91, 92 and 93, including magnetic bodies, are attracted to the fields 95. Accordingly, even when no electrical power is applied to the DC motor, a torque is generated in such a direction as to rotate the rotor 94. The torque rotates the rotor 94 so that the magnetic poles 91, 92 and 93 are stabilized in the magnetic field that has been generated by the fields 95. The torque that is going to rotate the rotor 94 is generated by the attraction between the magnetic poles 91, 92 and 93 and the fields 95. Thus, the angle of rotation of the rotor 94 at which the magnetic poles 91, 92 and 93 are stabilized changes with the positional relationship between the fields 95 and the magnetic poles 91, 92 and 93.
The xe2x80x9cstabilized statexe2x80x9d normally refers to a state in which one of the magnetic poles 91, 92 and 93 is closest to one of the fields 95. For example, FIG. 8A illustrates one of those stabilized states in which the magnetic pole 91 is closest to the N pole field 95. In such a state, there is no torque that is going to rotate the rotor 94.
Suppose the rotor 94 is rotated clockwise from the position shown in FIG. 8A. In that case, when the rotor 94 is rotated 60 degrees from the position shown in FIG. 8A, the magnetic pole 92 will be closest to the S pole field 95 as shown in FIG. 8B. This is another stabilized state. Since the rotor 94 has the three magnetic poles 91, 92 and 93 and the number of the fields 95 is two, there will be six stabilized states for one rotation of the rotor 94. That is to say, every time the rotor 94 rotates 60 degrees, one of the six stabilized states appears.
FIG. 9 shows the magnitudes and directions of the torques that are generated by the magnetic attraction between the rotor 94 and the fields 95. In FIG. 9, the state shown in FIG. 8A is regarded as an initial state. If a force is externally applied to the rotor 94 to rotate the rotor 94 clockwise from the initial state as indicated by the point A in FIG. 9 with no electrical power applied to the DC motor, the attraction between the N pole field 95 and the magnetic pole 91 generates a torque in the direction opposite to the rotational direction. As the angle of rotation increases, this reverse torque increases its magnitude. And when the rotor 94 rotates approximately 15 degrees, the magnitude of the reverse torque is maximized as indicated by the point B in FIG. 9. As the rotor 94 is further rotated, attraction is soon generated between the magnetic pole 92 and the S pole field 95. Accordingly, the reverse torque applied to the rotor 94 decreases gradually. And when the rotor 94 rotates approximately 30 degrees, no reverse torque is applied to the rotor 94 anymore as indicated by the point C in FIG. 9.
As the rotor 94 is further rotated, the attraction between the magnetic pole 92 and the S pole field 95 dominates, thereby generating a torque that rotates the rotor 94 clockwise. When the rotor 94 rotates approximately 45 degrees, the torque that rotates the rotor 94 clockwise is maximized as indicated by the point D in FIG. 9. But that torque also decreases as the rotor 94 is further rotated. And when the rotor 94 rotates approximately 60 degrees, no torque that rotates the rotor 94 clockwise is applied to the rotor 94 anymore as indicated by the point E in FIG. 9.
Actually, though, a friction torque is applied to the shaft of the rotor 94 as indicated by the one-dot chains in FIG. 9. Accordingly, unless a torque that has a magnitude greater than that of the friction torque is generated and applied to the rotor 94, the rotor 94 never rotates. Thus, the effective torque applied to the rotor 94 is indicated by the solid curve in FIG. 9. As can be seen from FIG. 9, such a torque variation is repeatedly caused every time the rotor 94 rotates 60 degrees. As indicated by the solid curve in FIG. 9, the effective torque may be positive, negative or zero depending on the angle of rotation of the rotor 94. This effective torque is the so-called xe2x80x9ccogging torquexe2x80x9d.
If the DC motor is stopped by discontinuing the supply of power to the DC motor, the magnitude and the direction of the cogging torque change with the stop angle of the rotor 94. Accordingly, when the rotor 94 reaches such an angle as to generate zero cogging torque (e.g., approximately 30 degrees or approximately 60 degrees in the example shown in FIG. 9), the rotor 94 can be stopped without being affected by the cogging torque.
However, if the rotor 94 should be stopped at such an angle as to generate a positive cogging torque, then that cogging torque is applied to the rotor 94, thereby rotating the rotor 94 excessively (i.e., to an angle greater than the desired angle) until the rotor 94 is stabilized. In the example shown in FIG. 9, the rotor 94 is rotated unintentionally to around 60 degrees, around 120 degrees, etc. On the other hand, if the rotor 94 should be stopped at such an angle as to generate a negative cogging torque, then a cogging torque is generated and applied to the rotor 94 in such a direction as to rotate the rotor 94 in the backward direction. In that case, just before the rotor 94 stops rotating, the rotor 94 rotates in the backward direction until the rotor 94 is stabilized. In the example shown in FIG. 9, the rotor 94 retrogrades to around 0 degrees, around 60 degrees, etc. For these reasons, when a DC motor is used, it is difficult to control the stop angle of the rotor 94 accurately enough.
Furthermore, a non-uniform torque is generated around the shaft of a DC motor when power is supplied to the DC motor. A DC motor of a small size, in particular, has a small number of magnetic poles, and therefore, there is a significant variation in the torque generated during one rotation of the rotor 94. The output of a DC motor is also affected by a variation in the power supplied to the DC motor to drive it or in the load connected to the motor. Consequently, there is a great variation in the output of the DC motor.
In addition, there is also a great variation in the load applied to a DC motor (e.g., friction caused at the bearing thereof). In particular, a load variation resulting from the difference between static friction and kinetic friction is a problem. More specifically, when power is supplied to a DC motor, the static friction caused at the bearing increases proportionally to the torque generated at the rotor 94. However, once the rotor 94 has started to rotate, the static friction changes into kinetic friction. Accordingly, the friction that interferes with the rotation of the rotor 94 decreases steeply. Such a variation in friction may be regarded as a sort of negative resistance. Thus, in performing a proportional control on a DC motor, such a variation introduces instability into the system. Consequently, it is particularly difficult to rotate the motor stably at a low velocity.
The rotor has a great moment of inertia, which poses another serious problem. In a DC motor of a small size, permanent magnets are used as its fields to cut down the space for the fields, and a rotor having a relatively large diameter may be used by making use of the extra space. In this manner, a high-efficiency and high-output motor is achievable. However, the larger the diameter of the rotor, the greater moment of inertia the rotor has. The equivalent mass of the moment of inertia of a rotor is changeable with the type of the load to be driven by a DC motor, but typically several times as great as the mass of the load to be driven by the DC motor.
As the moment of inertia of a rotor increases, it takes a longer and longer time to start or stop a DC motor. Accordingly, the load being driven by the DC motor cannot move so quickly for a while after the DC motor has been started. Likewise, it is also difficult for the DC motor to stop the load the instant the power that has been supplied to the DC motor is stopped.
These problems may be solved if the velocity of the load being driven by the DC motor is detected to position the load accurately. However, when such a control mechanism is added to detect the velocity of the load, the cost of a DC motor controller increases. Accordingly, a velocity detecting mechanism like that cannot be added to an apparatus that should be manufactured at a low cost.
Examples of controllers using a DC motor include an optical disk drive. For example, Japanese Laid-Open Publication No. 2000-20974 discloses a technique of pulse-driving a DC motor in an optical disk drive. An optical disk drive needs to move an optical head (i.e., the object of control) to a target location at a high speed and position the head accurately. Hereinafter, the conventional optical disk drive disclosed in Japanese Laid-Open Publication No. 2000-20974 will be described.
FIG. 10A is a block diagram illustrating the main section of an optical disk drive 101 that uses the conventional motor controller, while FIG. 10B is a plan view thereof. The optical disk drive 101 is a CD-ROM drive for use to read an optical disk (i.e., a CD-ROM in this case) 102, on which spiral tracks are formed. The optical disk drive 101 includes optical head (optical pickup) 103, optical head moving mechanism (see FIG. 10B), control unit 109, tracking error signal generator 121, tracking servo circuit 122, sled servo circuit 123 and comparator 124. The optical head 103 can be moved by the optical head moving mechanism in the radial direction of the optical disk 102 mounted (i.e., the direction indicated by the arrow A in FIG. 10B). The radial direction of the optical disk 102 will be herein simply referred to as a xe2x80x9cradial directionxe2x80x9d.
The optical head 103 includes an objective lens (or condenser lens) 132 and a tracking actuator 141. The objective lens 132 is movable both in the radial direction and in a direction parallel to the axis of rotation of the optical disk 102 (which direction will be herein simply referred to as a xe2x80x9crotation axis directionxe2x80x9d). The tracking actuator 141 moves the objective lens 132 in the radial direction (i.e., toward the inner periphery or the outer periphery of the optical disk 102). When a predetermined voltage is applied to the tracking actuator 141 by way of a driver 142, the tracking actuator 141 moves the objective lens 132 in the radial direction in accordance with the polarity and amplitude of the voltage.
The optical head moving mechanism includes sled motor (or feed motor) 107, driver 171 for driving the sled motor 107, lead screw (worm gear) 181 secured to the shaft 108 of the sled motor 107, worm wheel 241, pinion gear 242, rack gear 115 and a pair of guide shafts 116 for guiding the optical head 103 thereon. The optical head 103 is supported by the guide shafts 116 so as to be movable on the shafts 116. When the sled motor 107 is started by a driving control technique to be described later, the optical head 103 starts to move on the guide shafts 116 in a predetermined direction.
The control unit 109 is normally a microprocessor (or CPU) to perform an overall control on the optical head 103, sled motor 107, tracking servo circuit 122, sled servo circuit 123 and other components of the optical disk drive 101. This control unit 109 includes a pulse generator (or pulse voltage generator) 191. The control unit 109 and the comparator 124 together make up a shift detector for detecting the shift of the objective lens 132.
In this optical disk drive 101, the output voltage signal of the optical head 103 is input to the tracking error signal generator 121, which generates a tracking error signal TE as a voltage signal. The tracking error signal TE is input to the tracking servo circuit 122, thereby generating a tracking servo signal TS as another voltage signal. The level (or the voltage value) of this tracking servo signal TS represents the magnitude and direction of the shift of the objective lens 132 from its home position in the radial direction.
The tracking servo signal TS is input not only to the tracking actuator 141 by way of the driver 142 but also to the sled servo circuit 123. In response to the tracking servo signal TS, the tracking actuator 141 is driven so as to move the objective lens 132 toward the center of the target track. That is to say, a tracking servo control is performed.
However, it is still difficult for the objective lens 132 to follow the track accurately just by driving this tracking actuator 141. For that reason, the sled motor 107 is also driven to move the optical head 103 itself in the direction in which the objective lens 132 has moved. In this manner, a sled control is carried out so as to move the objective lens 132 back to its home position.
In response to the tracking servo signal TS, the sled servo circuit 123 generates a sled servo signal SS. The level (or the voltage value) of this sled servo signal SS represents the magnitude and direction of the shift of the objective lens 132 from its home position in the radial direction. The sled servo signal SS is input to the comparator 124, which digitizes the signal SS. Then, the output digital signal (or voltage) of the comparator 124 is input to the control unit 109.
FIGS. 11A, 11B and 11C are timing diagrams showing the respective waveforms of the sled servo signal (or voltage) SS, the output signal (or voltage) of the comparator 124 and the output signal (or voltage) of the pulse generator 191 in the optical disk drive 101 that uses the conventional motor controller. FIG. 12 is a flowchart showing how the control unit 109 performs the sled control operation.
As shown in FIGS. 11A and 11B, if the sled servo signal SS has a level (or voltage value) equal to or higher than a threshold level (i.e., a reference voltage value), the output signal of the comparator 124 is high (H). On the other hand, if the sled servo signal SS has a level (or voltage value) lower than the threshold level (i.e., the reference voltage value), the output signal of the comparator 124 is low (L).
In the optical disk drive 101, when the output signal of the comparator 124 rises from the L level to the H level, the shift of the objective lens 132 from its home position is regarded as having reached a certain limit. Then, the pulse generator 191 of the control unit 109 generates and outputs a pulse signal (or pulse voltage) having a predetermined pattern.
As shown in FIG. 11C, the pattern of the pulse voltage generated by the pulse generator 191 is predefined in such a manner that when the sled motor 107 is driven responsive to the pulse voltage, the motor 107 moves the objective lens 132 back to its home position. The output pulse voltage of the pulse generator 191 is applied to the sled motor 107 by way of the driver 171. In response to the pulse voltage, the sled motor 107 is driven, thereby moving the optical head 103 in the direction in which the objective lens 132 has moved and getting the objective lens 132 back to its home position.
As also shown in FIG. 11C, each pulse voltage consists of first and second (positive and negative) pulse voltages 151 and 152 having mutually different polarities. The absolute value of the first pulse voltage 151 is sufficiently greater than that of the start voltage of the sled motor 107. As used herein, the xe2x80x9cstart voltagexe2x80x9d of the sled motor 107 is a minimum voltage that needs to be applied to the sled motor 10 to start it in the optical disk drive 101. In this case, the absolute value of the first pulse voltage 151 may be about 120% to about 170% of that of the start voltage of the sled motor 107. On the other hand, the absolute value of the second pulse voltage 152 is smaller than that of the first pulse voltage 151. In this case, the absolute value of the second pulse voltage 152 may be about 50% to about 90% of that of the first pulse voltage 151.
When these pulse voltages are applied to the sled motor 107 by way of the driver 171, the sled motor 107 starts, and then accelerates, its rotation responsive to the first pulse voltage 151 but is braked and stopped responsive to the second pulse voltage 152 so as to get the objective lens 132 back to its home position.
Next, it will be described with reference to FIG. 12 how the control unit 109 performs its control over the sled motor 107. First, in Step 201 shown in FIG. 12, the control unit 109 determines whether or not the output signal (or voltage level) of the comparator 124 has changed from L level into H level. If the answer to the query of Step 201 is NO, then the pulse generator 191 outputs no pulse signal (or pulse voltage) in Step 202. In that case, the processing returns to Step 201 to start the processing steps all over again.
On the other hand, if the answer to the query of Step 201 is YES, then the pulse generator 191 generates and outputs the pulse signal (or pulse voltage) in Step 203 as described above. Thereafter, the processing returns to Step 201 to start the processing steps all over again. In this manner, even if any variation has been caused in the load of the optical head moving mechanism, the sled motor 107 still can be rotated and driven stably and accurately.
Although the rotor of the DC motor has a great moment of inertia, the conventional optical disk drive described above can stop the rotor quickly, thus increasing the accuracy of control to a certain degree. The conventional optical disk drive, however, has the following drawbacks.
Firstly, it is difficult for the conventional optical disk drive to control the rotation of the DC motor at a sufficiently small step angle. Thus, the conventional optical disk drive cannot achieve desired high positioning accuracy. To achieve a high resolution by minimizing the step angle, the pulse voltage applied to the DC motor to drive it may have its amplitude (i.e., the level) or pulse width reduced. However, if the level of the pulse voltage is decreased, then the DC motor will be affected by the variation in load or torque more easily, thus making it difficult to minimize the step angle.
On the other hand, if the pulse width is shortened, then the motor might just vibrate but not rotate at all at a certain angle of rotation or less. This phenomenon is brought about by the cogging torque. Specifically, even if the rotor of the motor has rotated to just a small degree by applying the pulse voltage with such a short width thereto, the cogging torque, which rotates the rotor to the opposite direction, is generated and applied to the rotor the instant the drive voltage reaches zero. Thus, the rotor returns to its original position. Accordingly, no matter how many times the same short pulse is applied repeatedly, the rotor will not rotate.
This phenomenon becomes even more noticeable if the driving mechanism that connects the motor to the object of control gets loose and unfixed or exhibits some elasticity due to the deformation. In that case, the motor just vibrates and cannot move the object of control at all. Accordingly, once such a phenomenon has occurred, it is virtually impossible to minimize the step angle by decreasing the amplitude of the pulse voltage or by shortening the pulse width thereof. Thus, the conventional control technique cannot increase the positioning accuracy except a very limited situation.
Furthermore, if the braking pulse is applied to the motor just after the driving pulse has been applied thereto, then the stability of control might be decreased significantly. Also, the load of a driving mechanism is not always the same both in the forward and backward directions. Rather, when the step angle is that small, the load in the forward direction is often different from the load in the backward direction. A difference like that is particularly remarkable when the driving mechanism that connects the motor to the object of control has low rigidity.
For example, some motor controller might get the object of control pressed by a compressed spring so to speak if the controller continues to drive the motor in the same direction. In that case, the load is very heavy for the motor in the forward direction (i.e., the direction in which the motor should move the object) but is extremely small in the backward direction. If a braking pulse, which is smaller in amplitude than the driving pulse by a predetermined percentage, is applied to the motor in such a state, then the motor will rotate in the backward direction. In the worst-case scenario, even the object of control also moves backward, thus deteriorating the stability of control considerably. Accordingly, the braking pulse should have its voltage value and pulse width defined in such a manner as to eliminate that phenomenon. Thus, it is rather imaginable that a braking pulse having such limited voltage value and pulse width is effective insufficiently against the great moment of inertia of the rotor.
In the conventional control technique described above, the amplitude of the pulse voltage applied to the DC motor is about 1.7 times as high as that of the minimum voltage that needs to be applied to the DC motor to start it, thereby attempting to decrease the step angle as much as possible. Also, if the object has moved to less than the predetermined magnitude responsive to the pulse voltage that has been applied to the motor a single time, the same pulse voltage is supposed to be applied a number of times.
However, if the predetermined magnitude of movement is not achieved upon the single application of a pulse voltage having such a small level, then it is highly probable that the object will not move, either, no matter how many times the same small pulse voltage is applied to the motor. The reason is as follows. For example, in the above-described situation where the object of control is getting pressed by a compressed spring so to speak, the slight movement of the object of control further compresses the spring. Thus, in that case, a heavier load will be constituted by the object of control once the pulse voltage has been applied to the motor.
In order to overcome the problems described above, an object of the present invention is to provide a motor controller that can perform a stabilized and accurate positioning control using a DC motor and a method of driving a DC motor.
Another object of the present invention is to provide a disk drive including such a motor controller.
A motor controller according to a preferred embodiment of the present invention includes a DC motor, a driving mechanism, voltage generating means and control means. The driving mechanism is provided to move an object having a predetermined mass against a load applied to the object and by transmitting a rotational force of the DC motor to the object. The voltage generating means generates first and second voltages. The first voltage has amplitude high enough to rotate the DC motor to such a degree as to get the object moved by the driving mechanism, while the second voltage has the same polarity as the first voltage and has such amplitude as to prevent the DC motor from rotating either in a backward direction due to a cogging torque or in a forward direction. The control means controls the voltage generating means in such a manner that the voltage generating means applies the second voltage to the DC motor after having applied the first voltage to the DC motor.
In one preferred embodiment of the present invention, the control means preferably gets a pulse voltage applied as the first voltage to the DC motor. A pulse width T of the pulse voltage preferably satisfies txe2x89xa6Txe2x89xa65t, where t is an electrical time constant of the DC motor.
In another preferred embodiment of the present invention, the first voltage is preferably three times or more as high as a minimum voltage that needs to be applied to the DC motor for the driving mechanism to overcome the load and move the object.
In this particular preferred embodiment, the control means preferably senses the magnitude of movement of the object. If the control means senses that the object has moved to less than a predetermined value, the control means preferably gets the first and second voltages repeatedly applied from the voltage generating means to the DC motor.
More particularly, the control means preferably senses the magnitude of movement of the object. If the control means senses that the object has moved to less than the predetermined value, the control means preferably gets the pulse width T of the pulse voltage increased.
In still another preferred embodiment, the voltage generating means preferably generates the first and second voltages that are both positive or both negative. The control means preferably controls the voltage generating means in such a manner that the voltage generating means selectively applies the positive first and second voltages or the negative first and second voltages to the DC motor. The first and second voltages preferably change their polarity from positive into negative, or vice versa, depending on a direction in which the object should be moved.
In yet another preferred embodiment, the control means preferably controls the voltage generating means in such a manner that the voltage generating means alternately applies the first and second voltages to the DC motor.
In yet another preferred embodiment, the voltage generating means may include a pulse width modulator and may generate the first and second voltages as effective pulse voltages to be output from the pulse width modulator.
In yet another preferred embodiment, if the object is not driven for a predetermined amount of time or more, the amplitude of the second voltage may be decreased.
In yet another preferred embodiment, the voltage generating means preferably includes switching means and generates the first voltage by turning the switching means ON.
A disk drive according to another preferred embodiment of the present invention includes driving means, a head, a head moving mechanism, a DC motor, voltage generating means and control means. The driving means rotates a disk. The head reads and/or writes information from/on the disk. The head moving mechanism moves the head in a radial direction of the disk. The DC motor drives the head moving mechanism. The voltage generating means generates first and second voltages. The first voltage has amplitude high enough to rotate the DC motor to such a degree as to get the head moved by the head moving mechanism, while the second voltage has the same polarity as the first voltage and has such amplitude as to prevent the DC motor from rotating either in a backward direction due to a cogging torque or in a forward direction. The control means controls the voltage generating means in such a manner that the voltage generating means applies the second voltage to the DC motor after having applied the first voltage to the DC motor.
In one preferred embodiment of the present invention, the control means preferably gets a pulse voltage applied as the first voltage to the DC motor. A pulse width T of the pulse voltage preferably satisfies txe2x89xa6Txe2x89xa65t, where t is an electrical time constant of the DC motor.
In another preferred embodiment of the present invention, the first voltage is preferably three times or more as high as a minimum voltage that needs to be applied to the DC motor for the head moving mechanism to overcome a load applied to the head and move the head.
In still another preferred embodiment, the control means preferably controls the voltage generating means in such a manner that the voltage generating means alternately applies the first and second voltages to the DC motor.
In yet another preferred embodiment, the voltage generating means may include a pulse width modulator and may generate the first and second voltages as effective pulse voltages to be output from the pulse width modulator.
Another preferred embodiment of the present invention provides a method of driving a DC motor for the purpose of transmitting a rotational force of the DC motor to an object, having a predetermined mass, by way of a driving mechanism. The driving mechanism is coupled to the DC motor and moves the object against a load applied to the object. The method includes the step of (a) generating first and second voltages. The first voltage has amplitude high enough to rotate the DC motor, while the second voltage has the same polarity as the first voltage and has such amplitude as to prevent the DC motor from rotating either in a backward direction due to a cogging torque or in a forward direction. The method further includes the steps of (b) applying the first voltage to the DC motor and then (c) applying the second voltage to the DC motor.
In one preferred embodiment of the present invention, the step (b) preferably includes the step of applying a pulse voltage as the first voltage to the DC motor. A pulse width T of the pulse voltage preferably satisfies txe2x89xa6Txe2x89xa65t, where t is an electrical time constant of the DC motor.
In another preferred embodiment of the present invention, the step (b) preferably includes the step of applying the first voltage, which is three times or more as high as a minimum voltage that needs to be applied to the DC motor for the driving mechanism to overcome the load and move the object, to the DC motor.
In still another preferred embodiment, the method may further include the step of alternately applying the first and second voltages to the DC motor.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.